In recent years, a sound processing apparatus such as a microphone array has been manufactured for the purpose of installing the sound processing apparatus in a hands-free phone or the like. The sound processing apparatus performs a process of suppressing stationary noise included in an input sound. The stationary noise is a sound that is input to the sound processing apparatus from a plurality of directions. When a vehicle is taken as an example, the stationary noise corresponds to a sound (road noise) of a tire of the vehicle during traveling of the vehicle, a sound of air blown by an air conditioner installed in the vehicle, and the like. For example, as one technique for suppressing a sound, there is a synchronous subtraction method that enables a sound input from a specific direction to be suppressed. Although the sound that is input from the specific direction can be suppressed by the synchronous subtraction method, it is difficult to sufficiently suppress sounds (such as stationary noise) input from a plurality of directions in the synchronous subtraction method.
The sound processing apparatus uses a suppression processing method using a spectral subtraction scheme for processing an input signal on a frequency axis. When the suppression processing method is used, the sound processing apparatus uses a window function to perform a windowing process on an input signal subjected to a synchronous subtraction process and performs high-speed Fourier transform on the input signal subjected to the synchronous subtraction process so as to divide the input signal into a phase spectrum and a power spectrum. Then, the sound processing apparatus subtracts, from the power spectrum, a power spectrum that corresponds to stationary noise. After that, the sound processing apparatus performs inverse Fourier transform on the phase spectrum and the power spectrum and restores the signal so that the restored signal has the suppressed stationary noise. Since the sound processing apparatus uses the suppression processing method, the sound processing apparatus can obtain an excellent result of suppression of a component that corresponds to the stationary noise included in the input signal. For example, the suppression processing method that is performed using the spectral subtraction scheme is disclosed in International Publication Pamphlet No. WO2007/018293, Japanese Laid-open Patent Publication No. 2003-271191, and “Suppression of Acoustic Noise in Speech Using Spectral Subtraction” by Steve F. Boll, IEEE TRANSACTIONS ON ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, VOL. ASSP-27, NO. 2, APRIL 1979.